The sendmail program recognizes three signals that cause it to
perform certain actions. SIGINT causes sendmail to
clean up after itself and exit. Beginning with V8.7,
SIGHUP causes sendmail to
re-execute itself (thus restarting and reading its configuration
file anew). Also beginning with V8.7,
SIGUSR1 causes sendmail to log its file
descriptors and other information.

Whenever sendmail
gets a SIGINT signal (as would be the case if the system
were being shut down), it tries to exit cleanly.

First it unlocks any queued file it is processing. This has the
effect of canceling delivery so that the message will be tried
again when the system comes back up. Then sendmail resets its
identity to the identity it originally ran under.
This causes accounting records to correctly show that the
same user sendmail started as has exited. Finally,
sendmail exits with EX_OK, no matter what,
so that errors will not be produced during shutdown.

As a final note, beginning with V8.7,
SIGINT is ignored when sendmail is running
in rule-testing mode with -bt.

Beginning with V8.7, a SIGHUP signal will
cause sendmail to re-execute itself with its original
command line. This works only if it is running in daemon mode
(with -bd; see Section 36.7.5, -bd). For example,
consider initially running sendmail like this:

# /usr/lib/sendmail -bd -q1h

Then imagine that you changed something in the configuration file
and wanted the running daemon to reread that file. You could
cause that to happen by killing the currently running daemon with
a SIGHUP signal:

# kill -HUP `head -1 /etc/sendmail.pid`

This will cause sendmail to execute the command

/usr/lib/sendmail -bd -q1h

The original daemon exits, and the newly executed daemon
replaces it.

Be aware that this works only if you run sendmail
using a full pathname. If you use a relative path, an attempt
to restart sendmail with SIGHUP will fail, and the following warning
will be logged at LOG_ALERT:

could not exec bad command line here: reason

This is a very serious situation because it means that your
original daemon has exited and no new daemon ran to replace it.

Beginning with V8.6.5, sendmail responds to a
SIGUSR1 signal. This signal
causes sendmail to syslog
at LOG_DEBUG the several items that define its state.
[2]
That syslog output begins with a line that looks like this:

[2] This same information is syslog'd if the daemon looses
track of $j in $=w
and if $j becomes or is not fully qualified.

-- dumping state on reason: $j = val --

where reason can be any one of the following:
siguser1
The information has been logged because sendmail received
a SIGUSR1 signal. In this instance the daemon logs
the information and continues to run.

daemon lost $j

The information has been logged because a running daemon
discovered that the value in $j (the canonical name
of this host; see Section 31.10.20, $j)
disappeared from the class $=w (the list of all names
by which the local host is known; see Section 32.5.8, $=w).
This test is made and this information is logged only if sendmail was compiled with XDEBUG defined
(see Section 18.8.57, XDEBUG).
In this instance the daemon logs the information and aborts.

daemon $j lost dot

The information has been logged because a running daemon
discovered that the value in $j (the canonical name
of this host; see Section 31.10.20) was no longer
canonical (no longer contained a dot inside it).
This test is made and this information is logged only
if sendmail was compiled with XDEBUG defined
(see Section 18.8.57).
In this instance the daemon logs the information and aborts.

Whichever the reason, the information that is logged for each looks pretty much
the same; for example,

We have described the first line already. If for some reason $j
is missing from $=w, that line will be followed by

*** $j not in $=w ***

The second line simply shows the number
of children the daemon has forked and currently has out doing
other work in parallel with itself. That line is followed by three
sections of information. The first two sections are always output;
the third is output only if rule set debug_dumpstate exists.

When sending mail, outgoing connections are maintained for efficiency,
and information about those connections is cached.
Before connecting to a remote host, for example, sendmail checks its
cache to see whether that host is down. If it is, it skips
connecting to that host.

This output is highly detailed and very complicated. See the
-d11.1 debugging switch (Section 37.5.44, -d11.1)
for a full description.

If rule set debug_dumpstate
[3]
is defined in your configuration file, it will
be called here, and the above line of output will be printed. The stat
is the numeric representation of the code returned by
sendmail's internal rewrite() routine. That code will be
either EX_OK (0) if there were no parsing errors,
EX_CONFIG (78) if there were, or EX_DATAERR (65)
if there was a fatal error (such as too much recursion or a replacement
was out of bounds). Text describing the error is also logged and
will appear in this output.

[3] In V8.7 sendmail this is rule set 89. Beginning with
V8.8 sendmail, rule sets 80 through 89 are reserved
for use by vendors, such as Sun Microsystems.

Rule set debug_dumpstate
is called with an empty workspace. After rule set debug_dumpstate is
done, each token in the resulting new workspace is printed one per
line. This gives you a hook into the internals of sendmail,
enabling you to display information that might otherwise be invisible.
For example, consider the desire to display identd information,
the current sender's address, and the current queue identifier:

Sdebug_dumpstate
R$* $@ $&_ $&s $&i

Here, the $* in the LHS matches the zero tokens passed
to rule set debug_dumpstate. The $@ prefix in the RHS suppresses
recursion. Each of the three macros that follows is stated with
a $& prefix (see Section 31.5.3, "Use Value as Is with $&") that prevents
each from being prematurely expanded when the configuration file is first read.

Another example might involve the need to look up the current recipient's host
with DNS:

Sdebug_dumpstate
R$* $@ $[ $&h $]

The $[ and $] operators (see Section 28.6.6, "Canonicalize Hostname: $[ and $]")
cause the hostname appearing between them to be looked up with
DNS and replaced with its full canonical name. Again, the
macro h is prefixed with $& to prevent premature
expansion.

In general, rule set debug_dumpstate
should be excluded from your configuration file.
When a problem does appear, you can define it, restart the daemon, and
then wait for the problem to reoccur. When it does, kill sendmail
with a SIG_USR1 and examine the syslog result.

Do not be tempted to use rule set debug_dumpstate
for routine logging of specialty
information. Forcing rules to be processed with a signal is fraught with danger.
The current active rule set can, for example,
be clobbered in possibly unrecoverable ways. Use this
rule set debug_dumpstate
technique only to solve specific problems, then erase
it when the problem is solved.